In general, a spark plug includes a center electrode and a ground electrode. The center electrode is projected from a tip of an insulator in a state where the center electrode is retained by an axial hole of the insulator. On the other hand, the ground electrode is joined to a tip section of a metal shell (also called a metallic shell but hereinafter called the metal fitting) through a welding. When the ground electrode is welded to the metal shell, a welding sag is generated on a surrounding of its welded section. The welding sag is usually removed by a machining such as a blanking punch, as described in a pre-published document of a Japanese Patent Laid-open Application Publication (tokkai) No. 2011-175985 published on Sep. 8, 2011.
In recent years, in order to respond to a downsizing of an internal combustion engine and an improvement in a fuel consumption of the internal combustion engine, a reduction in a diameter of the spark plug has been advanced. In association with the reduction in the diameter of the spark plug, a radial clearance between the metal shell and the insulator becomes narrow so that there is a tendency of generating a, so-called, lateral flying spark (a spark is not generated in a regular spark discharge gap). Conventionally, in order to secure a radial directional clearance between the metal shell and the insulator, the welding sag on inner surface sides of the metal shell and the insulator is removed through the machining after the welding of the ground electrode. However, it is difficult to sufficiently remove the welding sag. Therefore, a task such that, in part of the spark plugs such as a high ignitability spark plug or so forth, it is difficult to secure a demanded clearance.